1. Field of the Invention
The present invention relates to gasket and seal materials, and especially gasket materials pre-formed into various shapes (e.g. an O-ring seal in O-ring groove flanges) for sealing an apparatus together to contain fluid flow therethrough, such as in a transformer radiator flange.
2. Description of Related Art
O-ring flanges are used frequently in the joining of piping or enclosures where a metal-to-metal contact between the components is required. In this flange design, a groove is machined into one flange face in such a way that a resilient elastomeric type gasket sits tight within it. The gasket and groove are designed so that ideal compression of the elastomeric gasket occurs with the two flanges brought together until they are contacting. With the two flanges contacting, there exists metal-to-metal contact of the flanges to allow for more of a load-bearing assembly, i.e., the metal flanges end up carrying the majority of the bolt load and external loads that may be exerted on this joint. In standard flat-face or raised-face flange designs, the flanges do not contact, and the entire bolt load and external load are carried by the gasket. The integrity of the joint is, thus, highly dependent on the gasket.
An example of one application of O-rings is in the radiator piping of large electric transformers. Transformers convert incoming voltage to either a higher or lower output voltage depending on its output. In the process, the transformers develop a significant amount of heat that must be continuously dissipated. The heat dissipation is typically accomplished through the use of special transformer oils which are circulated within the transformer and then cooled through externally-mounted, air-cooled radiators. For example, in a transformer converting 69 kV to 13 kV, heat transfer conditions require the use of about 6" diameter class 150 flanges and piping that connect to an externally mounted radiator and circulation pump. The design of the externally mounted radiator is such that it is hanging off of the side of the transformer. As such, there is a tremendous bending moment applied to several of the flanges. The presence of this bending moment has compelled use of O-ring joint flanges that are better suited, structurally, to carry this type of load.
A typical transformer O-ring seal comprises a ring of elastomer or cork/elastomer (e.g. butyl rubber, neoprene, ethylene-propylene diene monomer (EPDM), etc.) that is sized (ID, OD & thick) to fit snug within the groove on one flange and stand above the groove by about 20-40% of the depth.
Existing elastomer or cork/elastomer gaskets have several drawbacks. For instance, a sometimes difficult compromise must be struck between a material which provides a tight seal and a material which is adequately durable and chemical/heat resistant for long-term use. Another common constraint is that existing gaskets are not always able to compensate for, misaligned, bent, corroded, or otherwise defective flange faces.
It is important that the flanges in O-ring groove flanges are kept in contact with each other during service. Such contact is important for external load carrying purposes. With the flanges contacting and the O-ring captured within the groove, there is no allowance for gasket creep or set. In this regard, the proper sizing and selection of the O-ring material is crucial. The gasket material must compress enough to develop and maintain enough counterforce to seal between the flanges when brought together; additionally, the O-ring material cannot cold flow or further compact with time, nor can it fracture under high loads.
Typically, elastomer or elastomer containing gasket materials are used as the seal in O-ring joint flanges as a material is needed that, once compressed, continues to exert a counterforce against the two flanges (i.e. exhibit resiliency) and, thus, maintains the seal. In applications such as transformer radiator flanges, cumulative thermal and chemical degradation of the elastomeric gasket while under compressive stress results in an inability of the gasket to maintain sufficient counterforce, thus resulting in premature leakage around the gasket. Materials such as elastomers are desired from the standpoint of resiliency and compressive counterforce, however, their long-term performance limitations when in the presence of elevated temperatures, ultraviolet radiation and transformer oils, make them less than ideal for applications such as transformer radiator flanges. What is required is a sealant that also exerts and maintains a compressive counterforce, yet is unaffected by thermal or chemical exposure.
One material that has superior heat and chemical resistant properties is polytetrafluoroethylene (PTFE). As a gasket, PTFE has exhibited utility as a material for use in harsh chemical environments which normally degrade many conventional metals, elastomers, and polymeric materials. Conventional, full density PTFE has a usable temperature range from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE gasket materials which have been compression molded or extruded and then heated to a temperature above 345.degree. C. exhibit poor mechanical properties, such as low tensile strength and low cold flow resistance. This limits or excludes the use of such materials in these applications requiring long term resistance to creep.
PTFE may be produced in an expanded porous form as taught in U.S. Pat. No. 3,953,566 issued Apr. 27, 1976, to Gore. Expanded polytetrafluoroethylene (ePTFE) is of a higher strength than conventional PTFE, has the chemical inertness of conventional PTFE, and has an increased temperature range of up to 315.degree. C. in service. An example of a porous expanded PTFE gasket material is available from W. L. Gore & Associates, Inc., of Elkton, Md., under the trademark GORE-TEX Joint Sealant.
Although polytetrafluoroethylene (PTFE) can provide the necessary durability, its tendency to experience compressive creep renders this material problematic as well. PTFE gaskets (virgin, filled or expanded) all exhibit varying degrees of compressive creep or flow. With metal-to-metal contact of the flanges in an O-ring joint flange, there is no mechanism for compensating for even a slight amount of creep. If the gasket creeps and, as a result, becomes thinner, there is no longer a counterforce being exerted by the gasket against the flanges.
One suggestion for achieving the chemical resistance of PTFE but limiting the amount of creep of the material is to coat a generally creep-stable material such as synthetic rubber with a coating of PTFE to provide chemical resistance. One example of such a structure is presented in U.S. Pat. No. 4,898,638 issued Feb. 6, 1990, to Lugez. In this patent it is taught that through a disclosed process one or more films of only partially porous PTFE can be adhered to a rubber sheet to provide a gasket material with chemical resistance. While this approach may address some of the problems with existing O-ring materials, the PTFE film can crack under the stresses of compression, leading to exposure and failure of the core elastomer. Further, it is believed that longer life and better thermal and chemical resistivity are possible if an expanded PTFE material is employed throughout the O-ring.
As is disclosed in co-pending United States patent application Ser. No. 050,903, filed Apr. 20, 1993, it has been determined that a PTFE sealing material can be produced with limited long-term creep by wrapping a core of elongated or expanded PTFE with a high strength film of expanded PTFE. The high strength film is resistant to deformation and stretching and serves to contain the PTFE core in place within a compressed gasket. This material has proven to be quite effective in sealing plate and frame heat exchangers--providing thermal and chemical protection, long-life and durability, and ease in replacement. However, it is believed that with modifications such material may be useful in other sealing environments.
Accordingly, it is a primary purpose of the present invention to provide a gasket material for an O-ring groove flange apparatus that provides an effective long-term seal under pressure, while being durable, chemical and thermal resistant, non-contaminating, and easy to install.
It is still another purpose of the present invention to provide a gasket material for O-ring groove flanges that provide the benefits of PTFE or expanded PTFE material, while avoiding the problem of creep.
It is a further purpose of the present invention to provide a method for making and optimally using a gasket material with the above properties.
These and other purposes of the present invention will become evident from review of the following specification.